US scientists have discovered a gene that appears to contribute to some cases of Tourette's syndrome.
Many genes may be involved in the syndrome
The gene sits on chromosome 13 and was found in a young child with the syndrome, Science reports.
When the Yale team checked another 174 unrelated people with Tourette's to see if they too carried this DNA, two had exactly the same SLITRK1 gene.
They stressed their discovery would only help explain a minority of the one in 100 people who have Tourette's.
Affects one in 100 people
Often more severe in boys than girls
Runs in families
Symptoms include tics - involuntary movements, which can also be vocal
Associated with psychological symptoms such as obsessive-compulsive or attention/hyperactivity behaviours
Experts know there is a strong family pattern suggesting the condition is at least partly inherited.
However, although the child of a person with the inherited form of the condition stands a 50% chance of inheriting the culprit gene or genes, this does not necessarily mean that they will develop symptoms.
Boys who have the DNA responsible for TS are more likely to develop symptoms than girls.
TS may also appear out of the blue, when it is known as sporadic TS.
The symptoms of the syndrome vary in severity from mild to severe. The common one is repetitive, rapid, sudden movements called tics, which can be verbal as well as physical.
People with TS may also show symptoms of other psychological disorders, such as obsessive-compulsive and ritualistic behaviour, attention deficit hyperactivity disorder (ADHD), sleep disorders, or learning disabilities.
Dr Matthew State, lead author of the study, said: "This finding could provide an important clue in understanding Tourette's on a molecular and cellular level.
"Confirming this, even in a small number of additional Tourette's syndrome patients, will pave the way for a deeper understanding of the disease process."
Dr Dorothy Taylor, professor of child and adolescent psychiatry and medical advisor to Tourette Scotland support group, said: "It's really interesting.
"It's undoubtedly a genetic condition in many cases, but there are probably several different genes involved.
"This could well help with identifying different forms of Tourette's. it is a complicated condition that is very varied."